1. Field of the Invention
The present invention relates to a method for the transmission of digital data wherein data are transmitted by m permitted discrete phase values by phase modulation of a carrier frequency.
2. Discussion of Background
Phase modulation, briefly called PSK (Phase Shift Keying) in the text which follows, is a transmission method in which the information is accommodated in the absolute phase of a carrier oscillation. By its nature, such a modulation method is little suited for communication channels in which rapid phase fluctuations or large phase shifts occur.
In general, disturbances in the channels used, which are restricted to a narrow frequency band, can be effectively counteracted by pseudo-randomly changing the carrier frequency in steps over a relatively large set of frequencies. In this method, briefly called FH (Frequency Hopping), a particular number of data bits are transmitted on each carrier frequency before the system skips to the next carrier frequency.
Since FH usually entails a random change of carrier phase during the change of the carrier frequency, which can be caused either by the modulator used or by the channel itself, it is found that, under normal circumstances, PSK and FH are not compatible, since the random phase change is an obstacle to coherent demodulation.
Nevertheless, the literature (Simon, M.K. et Polydoros, A., IEEE Transactions On Communications, Vol. COM-29, No. 11, Nov. 1981, p. 1644 ff) contains investigations conducted on a combined FH/PSK system without, however, discussing the technical implementation of coherent demodulation, that is to say, of the synchronization between transmitter and receiver.
In the Swiss Patent Application No. 3068/85-1, a FH/PSK method has been proposed in which, for the purpose of synchronizing the receiver, all modulation symbols are transmitted several times in a particular manner, the symbols being repeated in transmission sections having different carrier frequencies.
From the multiple transmission of the symbols, detailed information on the position of the carrier phase in each transmission section assigned to a carrier frequency can then be obtained.
Compared with non-coherent transmission methods, the FH/PSK method with coherent demodulation has significant advantages. These advantages are retained even in the presence of disturbance sources if the influence of the disturbance sources or their presence can be detected and taken into account.